1. Field of the Invention
The present invention relates to a packaging arrangement for electronic and/or optoelectronic assemblies and, more particulary, to such a package which advantageously utilizes a multi-layered ceramic (MLC) integrated circuit subassembly member.
2. Description of the Prior Art
Hybrid packages are defined as those packages which are used to house one or more integrated circuits, or chips, as well as other discrete components. These other components may comprise optoelectronic devices such as lasers, LEDs, or photodiodes. There are often many interconnections between devices inside a hybrid package, as well as many connections between the package and external components (e.g., input/output signals, power and ground supplies, test points). Hybrid packages may be classified into two major types: metal and ceramic.
Metal hybrid packages (often referred to as "bathtub packages") are known to be mechanically very rugged and provide inherent electro-magnetic interference (EMI) shielding, good thermal dissipation, and impedance-matched feed-throughs. This type of package may take on any desired configuration and is produced using a variety of standard techniques including casting, stamping, brazing, forming and injection molding. In spite of these advantages, a number of problems exist with the all-metal hybrid package. First, when used for housing a hybrid intergrated circuit (HIC)--the most prevalent application--an epoxying or soldering operation is required to attach the HIC to the bottom of the package. Issues regarding thermal mismatch of the metal package, attachment material (usually an epoxy) and the HIC substrate (usually a ceramic) may arise, where the mismatch may degrade the ability of the metal to transfer away the heat generated by the HIC. Additionally, this type of package requires the use of a large number of wirebonds to connect the various components inside the package, as well as to connect the HIC(s) to the leads which exit the package. It is well-known that the reliability of a package device is directly proportional to the number of wirebond attachments, where a large number of such bonds significantly reduces reliability. Wirebonds attached to incorrect locations, missing wirebonds, broken wirebonds, and shorted wirebonds are just a few examples of the known failure mechanisms. Further, for packages which must be hermetic, the type of sealant used to connect the external leads to the package may be a problem. Glass-to-metal seals are often used for this connection (see "Hybrid Packages Maximize Circuit Protection", Electronic Packaging and Production, January 1989, pp. 48 et seq.). In particular, the leads (usually a nickel/gold-plated nickel/iron alloy) are inserted through openings in the package (package base for through-the-board applications or package sidewalls for surface-mount applications), with a glass material used to fill the openings and surround the leads. Various glass materials are utilized to provide either a matched seal (coeffcient of thermal expansion of package, glass, and leads being matched) or a compression seal (coeffcient of thermal expansion of package&gt;glass&gt;leads), the latter providing a better hermetic seal. However, problems exist with these seals, especially when the leads are bent a number of times, or the package is subjected to extremes in temperature fluctuations. These conditions are known to cause cracks to develop between the leads and the glass sealant. These cracks may then propagate through the opening, resulting in a loss of hermeticity.
As an alternative to all-metal hybrid packages, a ceramic package, often referred to as a "flatpack" is used. In this design, a ceramic base, or a multi-layered ceramic (MLC) is utilized as the bottom support piece of the package, with a hybrid integrated circuit (HIC) attached to the base. Alternatively, the base may actually be the HIC itself. In the former case, the HICs may be connected to the MLC, with metallic paths formed on the various layers of the MLC used to replace a large number of the wirebonds required in the all-metal package. Thus, the ceramic package offers an improvement in reliability over the metal package by reducing the number of wirebonds. The sidewalls of the ceramic package comprise a metal which is chosen to match as closely as possible the physical properties (e.g., coeffient of thermal expansion) of the ceramic base. In most cases, a nickel-iron cobalt alloy, known by the trade name kovar.RTM. of Carpenter Technology, is used for this purpose. Major drawbacks associated with this type of package are its lack of protection from electro-magnetic interference (EMI) and mechanical fragility. This lack of protection in the ceramic package limits its usefulness to applications where neither EMI nor mechanical ruggedness are of great concern.
In the light of the above, a need remains in the prior art for a packaging scheme which has increased reliability over metal schemes, while also providing improved EMI shielding over known ceramic arrangements.